The present invention relates to a method for measuring a concentration of nonvolatile contents of an electrodeposition paint.
Automobiles and building articles are generally coated by electrodeposition coating. The electrodeposition coating line comprises, aside from an electrodeposition bath for immersing an article to be coated (hereinafter, referred simply to as "coating article"), a paint circulating means and other attached facilities, and the electrodeposition bath is, as a matter of fact, filled with an electrodeposition paint. An electrodeposition paint contains nonvolatile components such as a pigment and water-based resin (a water-soluble or water-dispersible resin) in a solution composed of a considerable amount of water and a certain amount of an organic solvent, and when the coating article is immersed in the electrodeposition bath and an electric voltage is applied, the pigment and resin are deposited (electrodeposit) on the coating article by electrophoresis and the coating is achieved. The organic solvent is used, meanwhile, for improving gloss or smoothness of the coating film.
As the coating progresses, the pigment and resin in the bath are consumed, and the concentration of nonvolatile contents lowers, and it is hence necessary to supplement the consumed portion. In order to compensate for the consumption, it is required to know the concentration of nonvolatile contents in the electrodeposition bath. Hitherto, because no method was known for easily measuring the concentration of nonvolatile contents, there has been employed a bath control method which comprises supplementing the paint in proportion to the approximate coating area of the coating article, or supplementing a specific volume of paint at specific time intervals; thereby, compensating for the consumption of the pigment and resin. Such a bath control method is likely to cause fluctuations in the concentration of nonvolatile contents in the electrodeposition bath.
Moreover, deposited amounts of pigment and resin in coating are not always proportional to a pigment concentration and resin concentration in the bath, and if not proportional, as the coating progresses, the ratio of pigment to resin continues to change. Further, mass sediment may be formed due to insufficient agitation in the bath or impurities brought into the bath from outside which may consequently cause variation of the ratio of pigment to resin. In this way, the concentration ratio of pigment to resin in the bath is often deviated largely from an initial ratio in the bath in the course of coating. In such a case, it is required to know the ratio of the pigment to resin in the concentration of nonvolatile contents, when returning to the initial ratio in the bath by supplementing the pigment and resin.
In an electrodeposition paint, since the pigment and resin are present as nonvolatile contents in the bath control as mentioned above, the nonvolatile contents were, conventionally, measured occasionally by manual work apart from a production line.
The concentration of the total nonvolatile contents totaling the resin and pigment components was measured in the following method of evaporating to dryness.
A specific amount of an electrodeposition paint is placed in a weighing dish, and is dried with heating for 1 to 3 hours, and the weight is measured before and after heating, and an amount of the nonvolatile contents is measured on the basis of this weight difference.
The ratio of the pigment component to resin component in the concentration of nonvolatile contents was also determined by measuring an ash amount according to the following procedure.
A specific amount of an electrodeposition paint is placed in a weighing crucible which has been dried for about 3 hours, and is heated for burning for about 60 minutes at 600.degree.-700.degree. C., and the weight is measured before and after the burning to determine the ash amount on a basis of a weight difference. The ash amount is an amount of the pigment, and a balance of subtracting the ash amount from the above amount of the nonvolatile contents is the resin content so that the ratio of the pigment to resin in the concentration of nonvolatile contents is known.
The conventional method, however, had the following problems.
First, it took a relatively long time in measuring the concentration of nonvolatile contents, although the measuring operation is easy. However, there are often a case of an electrodeposition paint where a component ratio in the concentration of nonvolatile contents (for example, the ratio of pigment content to resin content) does not vary so much, and a case where it is not required to know the ratio of components, but it is enough to know only a total concentration of the components. In these cases, a method of promptly and easily measuring the concentration of nonvolatile contents is keenly desired.
Second, in a case of measuring a ratio of pigment content to resin content in a concentration of nonvolatile contents, measurement of the ash amount is carried out together, but this measurement itself is difficult. For measuring the ash amount, a high temperature heat treating device must be used and careful handling for not scattering the ash is required so that the procedure, becomes difficult which requires an experienced skill. Hence, there is a strong demand for easily measuring the ratio of pigment content to resin content in the concentration of nonvolatile contents.